Collect static asset URLs from static_replace.replace_static_urls

Allows users to keep track of which static asset URLs were found in given text,
and what they were replaced from/to.

common/djangoapps/static_replace/__init__.py

@@ -148,7 +148,7 @@ def make_static_urls_absolute(request, html):
     )
 
 
-def replace_static_urls(text, data_directory=None, course_id=None, static_asset_path=''):
+def replace_static_urls(text, data_directory=None, course_id=None, static_asset_path='', static_paths_out=None):
     """
     Replace /static/$stuff urls either with their correct url as generated by collectstatic,
     (/static/$md5_hashed_stuff) or by the course-specific content static url
@@ -159,19 +159,29 @@ def replace_static_urls(text, data_directory=None, course_id=None, static_asset_
     data_directory: The directory in which course data is stored
     course_id: The course identifier used to distinguish static content for this course in studio
     static_asset_path: Path for static assets, which overrides data_directory and course_namespace, if nonempty
+    static_paths_out: (optional) pass an array to collect tuples for each static URI found:
+      * the original unmodified static URI
+      * the updated static URI (will match the original if unchanged)
     """
 
+    if static_paths_out is None:
+        static_paths_out = []
+
     def replace_static_url(original, prefix, quote, rest):
         """
         Replace a single matched url.
         """
+        original_uri = "".join([prefix, rest])
         # Don't mess with things that end in '?raw'
         if rest.endswith('?raw'):
+            static_paths_out.append((original_uri, original_uri))
             return original
 
         # In debug mode, if we can find the url as is,
         if settings.DEBUG and finders.find(rest, True):
+            static_paths_out.append((original_uri, original_uri))
             return original
+
         # if we're running with a MongoBacked store course_namespace is not None, then use studio style urls
         elif (not static_asset_path) and course_id:
             # first look in the static file pipeline and see if we are trying to reference
@@ -213,6 +223,7 @@ def replace_static_urls(text, data_directory=None, course_id=None, static_asset_
                     rest, str(err)))
                 url = "".join([prefix, course_path])
 
+        static_paths_out.append((original_uri, url))
         return "".join([quote, url, quote])
 
     return process_static_urls(text, replace_static_url, data_dir=static_asset_path or data_directory)